Phosphoric acid may be synthesized by the so called "wet process" technique wherein phosphate rock, for example, apatite, is reacted with sulfuric acid, of known concentrations, at elevated temperatures, generally above 70.degree. C. Existing "wet process" methods differ somewhat but have a common objective of producing a satisfactory phosphoric acid product yield together with a calcium sulphate crystal by-product and practically and efficiently separating the two compositions.
The reactions involved in the "wet process" synthesis of phosphoric acid evolve large volumes of carbon dioxide, and some hydrogen fluoride gas which result in the development of substantial amounts of foam. The foam produced hinders the reaction process by reducing the capacity of the equipment used in the various process steps and may result in overflows of foam which can create dangerous conditions for plant personnel because of the well-known caustic properties of phosphoric acid.
Various anti-foam agents have been used to control foaming in such processes, including tall oil fatty acids, oleic acids, sulfonated tall oil fatty or oleic acids, and silicones.
U.S. Pat. No. 3,437,437 to Dorwart discloses the use of the reaction product of hydroxylamine and a fatty acid to control foaming during the production of wet process phosphoric acid.
Canadian Pat. No. 803,928 also to Dorwart discloses that the effectiveness of the product of a hydroxylamine and a fatty acid can have increased anti-foam effectiveness by adding a surfactant to the mixture. An example of the surfactant used as an additive is 10% by weight of an alkyl aryl polyether alcohol based surfactant such as Triton X-100 which is commercially available from the Rohm & Haas Company. The use of the surfactant additive apparently acts to decrease the quantity of anti-foam ester which is required to control predetermined amounts of foam during processing of phosphate rock.
U.S. Pat. Nos. 4,065,402; 4,065,404; and 4,145,310 all to Satterwhite, et al. are directed to defoamers for high strength acid media. These patents disclose a defoamer for controlling foam in high strength acid media including wet process phosphoric acid manufacture media. The defoamers of these patents consist of sulfonated fatty acids such as tall oils combined with long chain alcohols such as dodecyl alcohol. Additionally, the U.S. Pat. No. 4,065,404 discloses the use of a non-ionic additive to the fatty acid long chain alcohol mixture. These non-ionic additives are preferably selected from the group consisting of tridecyloxypoly(ethyleneoxy ethanol and octylphenoxy (ethyleneoxy)ethanol.
U.S. Pat. No. 4,083,936 to Woodward discloses the use of phosphate esters of aliphatic alcohols as anti-foaming compositions in wet process manufacture of phosphoric acid to control the foam produced during phosphoric acid manufacture.
U.S. Pat. Nos. 3,653,827 and 3,761,423 to Hey, et al. the entire disclosures of which are incorporated herein by reference disclose the use of a defoaming agent which comprises a mixture of (1) a monoalkanolamide of a monoalkanolamine and a monocarboxylic acid having from 12 to 22 carbon atoms and a melting point below about 20.degree. C. and (2) a fatty acid which is a monocarboxylic acid having from 12 to 22 carbon atoms and a melting point below about 20.degree. C., in specifically defined proportions to increase the yield of phosphoric acid in the "wet process" synthesis and to inhibit the production of foam during such processes.
While the above-noted defoamers are apparently effective as defoamers during "wet process" manufacture of phosphoric acid, it has become apparent that the normal phosphate rock used in the phosphoric acid industry in the last few years is of a lesser quality than has been used before. The poorer quality phosphate rock causes an increase in the formation of foam during the wet process manufacture of phosphoric acid. To meet the increased foam problem an improved defoaming agent is required.